The present invention relates to a method for the thermal connection of overlapping connecting surfaces of two substrates, at least one substrate being transparent and laser energy being applied to the connecting surfaces from a rear side of the transparent substrate. The present invention further relates to a device for manufacturing a thermal connection of overlapping connecting surfaces of two substrates, at least one substrate being transparent and laser energy being applied to the connecting surfaces from a rear side of the transparent substrate, comprising a laser emitting device for emitting radiation energy and a radiation transmitting device for transmitting the radiation energy from the laser emitting device to a contact pair of connecting surfaces.
The term substrate in the present case is understood to include all components which are provided with a conductor structure and external connecting surfaces for contacting, i.e. chips as well as circuit boards, for example. A preferred area of application of the method proposed here or area of use of the devices proposed here lies in flip-chip technology and also in the field of SMD (surface mounted device) technology.
A method is known from JP 4-91 493 A in xe2x80x9cPatent Abstracts of Japanxe2x80x9d, 1992, in which, for the thermal connection of connecting surfaces of two substrates, energy is applied to the rear of a transparent substrate through a transparent pressing plate, by means of which the connecting surfaces of the substrates are brought into abutment. A transparent glass plate is used as the pressing plate. In this method, in contrast to the method known from DE 44 46 289 A1, the application of laser energy is not applied to all contact pairs of the connecting surfaces simultaneously, but rather sequentially, a plurality of contact pairs being combined to form a unit in each case, to which laser energy is applied as a whole.
In the known method, the use of the transparent glass plate does allow for the transmission of a contact pressure onto the connecting surfaces of the substrates forming contact pairs with one another. However, as a result of manufacturing discrepancies, which can result for example in different heights of the contact metallization constructed on the connecting surfaces, it may occur that a part of the opposing connecting surfaces has no contact, so that there is an insufficient introduction of thermal energy into the respective contact pairs, which can lead to contact failures with the possible consequence of component failure during operation of the substrates which have been contacted with one another.
In U.S. Pat. No. 4,978,835, the connecting surfaces of two substrates are brought into abutment by a glass plate or flexible, transparent membrane, the contact pressure being applied by a different gas pressure between the upper and lower sides of the glass plate or membrane, for example by a vacuum. Subsequently, energy is applied to the contact surfaces by a laser beam radiating through the glass plate or membrane.
It is the object of the present invention to propose a method and a device, which allows for improved contacting of connecting surfaces of two substrates and therefore increased reliability during operation of substrates of this type.
In the method according to the invention, laser energy is applied separately to each of the contact pairs constructed between two connecting surfaces of the opposing substrates. In this manner, the possibility is created of adjusting the beam path, more particularly the focusing of the laser beam in such a manner that an optimal application of energy for the respective contact pair is effected. In this respect, a focusing device is axially displaced as a function of angular movements of a swivel mirror unit. In this manner, it is possible to effect changes in the beam path, more particularly a lengthening or shortening of the beam path, without causing a defocusing of the laser beam in respect of the respective contact pair to which laser energy is applied.
As a result of the focusing of the laser beam onto the respective contact pair, laser energy is also only applied to a correspondingly reduced proportion of the substrate material, so that undesirable temperature stressing of the substrate material is kept to a minimum. Accordingly, the residual absorption capacity which is also present in a transparent substrate material is lower than in cases where laser energy is applied to the entire surface of the substrate material.
It has proved particularly advantageous if the radiation energy emitted by a laser emitting device is deflected in a beam path via a first swivel mirror device onto a second swivel mirror device and by the second swivel mirror device onto a contact pair. As a result of the above double deflection of the beam path, it is possible to position the laser emitting device, and the substrate arrangement comprising the two substrates, relative to one another as desired. More particularly, it is possible, proceeding from a stationary arrangement of the laser emitting device and substrate arrangement, to apply laser energy to each contact pair of the connecting surfaces in succession in a controlled manner, without having to change the positioning of the laser emitting device or the substrate arrangement.
In addition to the above-mentioned improvement in the contact quality between associated connecting surfaces of the substrates attained by separate application of laser energy to the individual contact pairs, an aspect of the method of the invention allows for a further solution of the object forming the basis of the invention.
According to one aspect of the method according to the invention, the connecting surfaces of the substrates are pressed against one another by means of a pressing device acting upon the substrates, a transparent, non-compressible and deformable volume being arranged between a transparent force introduction device of the pressing device and a rear side of a substrate. In this respect, the above-mentioned volume acts, as a result of its deformability, as a pressure cushion, which allows for an application of pressure to the respective substrate simulating fluid pressure. In this manner, in contrast to pressure application, it is possible by means of a rigid plate to produce deformations in the substrate which counteract the formation of gaps between connecting surfaces of contact pairs and can thus compensate height differences of contact metallizations arranged on the connecting surfaces caused by manufacturing discrepancies. If a measurement of the infrared radiation emitted by the contact pair is effected, in order to control the focusing of the beam path onto the respective contact pair, then it is possible to monitor the focusing in a simple manner. It is also possible, in order to control the focusing of the beam path onto the respective contact pair, to carry out an optical monitoring of the contact pair by means of a camera device, to which end a decoupling of the visible light from the beam path is effected. By using the same beam path both for the application of energy to the contact pair and for monitoring the correct focusing of the laser beam onto the contact pair, it is possible to effect the monitoring with minimal additional outlay in apparatus. Furthermore, as a result of the above-mentioned double use of the beam path, no optical transmission errors are possible, which would allow for discrepancies between the actual focusing and the focusing which is detected.
In order to control the focusing of the laser beam onto the contact pair, a method has also proved advantageous in which the beam path of a relatively weak pilot laser, which is permanently emitted, is superimposed over the beam path of the laser emitting device, which is constructed as a power laser device preferably operated in pulse mode, in order to allow for a control of the focusing of the beam path of the power laser by means of a control of the focusing of the beam path of the pilot laser. The advantage of this method consists in that there can be no focusing errors as a result of phase displacements of the emitted laser beam on account of the corresponding wave lengths of pilot laser and power laser. A laser diode installed in or on the laser emitting device can be used as a pilot laser. An infrared camera is particularly advantageously used as a camera.
A further, particularly advantageous variant of the method consists in effecting a monitoring of the contact pair by means of the camera device and a measurement of the infrared radiation emitted by said contact pair in order to control the effective laser capacity in the contact pair. This type of control has proved particularly precise, since it takes into account a parameter having a decisive effect on the result of the capacity control by measuring the infrared emission, namely the focusing of the contact pair. In this manner, it is possible, for example, to prevent insufficient laser capacity caused solely by a defocusing from being compensated by an increase in the laser energy, although in principle a correction of the focusing without increasing the laser energy would be sufficient in order to obtain the required energy input into the contact pair.
The device according to the invention comprises a radiation transmitting device for generating a beam path, which is deflected at least twice, from the laser emitting device to the contact pair with a first and a second swivel mirror device, a focusing device, which is axially displaceable in the beam path, being arranged in the beam path between the laser emitting device and the contact pair.
This special design of the radiation transmitting device allows for a stationary positioning of the laser emitting device and the substrate arrangement formed by the substrates which are to be contacted with one another, which positioning can be maintained during the successive contacting of all contact pairs of the connecting surfaces of opposing substrates. Furthermore, the above-mentioned design of the radiation transmitting device according to the invention allows for a separate application of energy to all contact pairs, so that the respective optimal beam path for obtaining a corresponding optimal contacting result can be adjusted for each contact pair. In order to compensate changes in the beam path, more particularly a lengthening or shortening of the beam path, caused by a rotation of the swivel mirror device, which can result in a defocusing of the laser beam at the contact pairs, the axially displaceable focusing device, which comprises a convergent lens for example, is provided in the beam path.
According to another aspect of the invention, a pressing device is provided, which acts upon the substrates and comprises at least one transparent force introduction device, which is provided at least in the contact region with a rear side of a substrate with a transparent, non-compressible and deformable volume. In this manner, a type of pressure cushion is formed between the force introduction device and the rear side of the substrate, which pressure cushion, as already explained above, allows for a fluid-like application of pressure to the substrate with corresponding substrate deformation.
It has proved particularly advantageous if the force introduction device is covered with a layer of plastics material in order to form the transparent, non-compressible and deformable volume.
In order to increase the efficiency of the transparent, non-compressible and deformable volume in respect of a fluid-like application of pressure to the rear side of the substrate, the volume can be provided with a deformation limiting device acting transversely to the pressing direction.
If the focusing device is arranged in the beam path between the first swivel mirror device and the laser emitting device, then defocusing caused both by the first swivel mirror device and by the second swivel mirror device can be compensated by the focusing device.
It has proved particularly advantageous if a radiation decoupling device is arranged in the beam path between the laser emitting device and the focusing device for decoupling and deflecting the visible light from the beam path into a camera device. In this manner, a direct optical monitoring of the focusing of the laser beam onto the respective contact pair is possible.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.